2B.5C: The Sediment Cell Model
The sediment cell concept (sources, transfers and sinks) is important in understanding the coast as a system with both positive and negative feedback, it is an example of dynamic equilibrium.
A sediment cell (or littoral cell) is a linked system of sources, transfers and sinks of sediment along a section of coastline.
Example:
Flanborough head - source region
Holderness coast - transfer zone
Spurn head - sink region
The coastline of England and Wales is divided into 11 primary sediment cells, with sub-cells within each primary cell. The boundaries are formed by major headlands or large estuaries.
A sediment cell operates as a closed system, with virtually no inputs or outputs of sediment from the cell.
This system contains inputs, transfers and outputs.
Inputs
Sources are places where sediment is generated, such as cliffs or eroding sand dunes. Some sources are offshore bars and river systems and these are an important source of sediment for the coast.
Some examples of sediment inputs are:
Transfers
Places where sediment is moving alongshore through longshore drift and offshore currents. (Drift-aligned) beaches and parts of dunes and salt marshes perform this function.
Some examples of sediment transfers are:
Sinks are locations where the dominant process is deposition and depositional landforms are created, including spits and offshore bars.
Some examples of sinks are:
A sediment cell (or littoral cell) is a linked system of sources, transfers and sinks of sediment along a section of coastline.
Example:
Flanborough head - source region
Holderness coast - transfer zone
Spurn head - sink region
The coastline of England and Wales is divided into 11 primary sediment cells, with sub-cells within each primary cell. The boundaries are formed by major headlands or large estuaries.
A sediment cell operates as a closed system, with virtually no inputs or outputs of sediment from the cell.
This system contains inputs, transfers and outputs.
Inputs
Sources are places where sediment is generated, such as cliffs or eroding sand dunes. Some sources are offshore bars and river systems and these are an important source of sediment for the coast.
Some examples of sediment inputs are:
- Cliff erosion,
- Onshore currents
- River transport
- Wind blown (aeolian) sediment from land
- Subaerial processes
- Marine organisms
Transfers
Places where sediment is moving alongshore through longshore drift and offshore currents. (Drift-aligned) beaches and parts of dunes and salt marshes perform this function.
Some examples of sediment transfers are:
- Longshore drift
- Swash
- Backwash
- Tidal currents
- Sea/ocean currents
- Wind (onshore, offshore or along shore)
Sinks are locations where the dominant process is deposition and depositional landforms are created, including spits and offshore bars.
Some examples of sinks are:
- Backshore depositional landforms
- E.g. sand dunes
- Foreshore depositional landforms
- E.g. beaches
- Nearshore depositional landforms
- E.g. bars
- Offshore depositional landforms
- E.g. barrier islands
Dynamic system
Sediment cells are dynamic because the sediment is constantly generated in the source region, transported through the transfer region and deposited in the sink region.
Dynamic equilibrium (in this instance) is reached when inputs of sediment from the source region are balanced by the amount being deposited in sinks. It's dynamic because although it's in balance, there's a constant movement of sediment through the system.
(Think of a classroom during the school day - always full of (roughly) the same amount of people, but the people in it change)
With a dynamic equilibrium, the size of the landforms in the transfer zone will remain the same. (But not the ones in the source and sink regions)
They may operate as complete circulations: Sediment is eroded from the depositional sink landforms and is carried offshore, being being re-transported onshore by currents and wind action that act at the source region.
However, the dynamic equilibrium is itself dynamic because its constantly changing as energy and sediment inputs constantly alter. The amount of stuff moving through is changing.
- E.g. climate change creating more frequent storms or erosion of the cliff line to a more resistant rock type.
- The system's equilibrium may be interrupted (e.g. during a storm event) but they tend to return to balance on average over time due to negative feedback.
- Seasonal change (e.g. storms and strong winds during winter) will change the dynamic equilibrium.
Coastal management in the source region may reduce sediment supply, e.g. sea walls preventing cliff erosion. Management in transport region may reduce or halt sediment supply to sink region, e.g. groynes trapping sediment to encourage beach outbuilding.
Feedback
Negative feedback: when the change produced creates effects that operate to reduce or work against the original change.
- E.g. when erosion leads to blockfall mass movement. The collapsed debris acts as a barrier protecting the cliff base, slowing or preventing erosion for a period of time.
- E.g. major erosion of sand dunes could lead to excessive deposition offshore, creating an offshore bar that reduces energy, allowing the dunes time to recover.
Positive feedback: when the changed produces an effect that operates to increase the original change.
- E.g. When wind erosion of a dune section during high velocity storms may removing stabilising vegetation.
- Further wind erosion now occurs in later low velocity wind conditions, increasing the depletion of dune sand.
A source region may be an eroding coastline. A sink region may be an outbuilding coastline.
Other
The volume of sediment generated, transferred and deposited is measured in the sediment budget.
Dynamic equilibrium can be considered over the temporal scales of: a storm event, one season, one year, a multi-decadal period of climate change (natural or anthropogenic)
Dynamic equilibrium can be considered over the temporal scales of: a storm event, one season, one year, a multi-decadal period of climate change (natural or anthropogenic)